The mechanism of charge transport through single molecules is a key issue in molecular electronics. In addition to tunneling transport, the thermally activated process plays an important role in some cases. This article introduces our experimental results on the transition of the charge transport mechanism from the tunneling to thermally activated process as a function of molecular length in π-conjugated molecular wires.
I describe the first principles nonequilibrium Green's function method for electric transport calculations in molecular junctions. Effects of electron-phonon scatterings were included by the conventional lowest order expansion formalism. Then importance of two concepts, “orbital engineering” and “anchor engineering”, was illustrated by theoretical calculations. Electron-phonon interactions in molecular junctions were also discussed by analysis of inelastic electron tunneling spectrum.
Two topics are presented about the magnetic properties of iron (II) phthalocyanine (FePc) adsorbed on metal substrates. The spin state and switching of magnetic anisotropy of FePc on Cu(110) and Cu(110)(2×1)-O are discussed based on the spin excitation spectra and the magnetic field evolution in the first topic. In the second topic, the formation of Kondo resonance state is described for FePc on Au(111) and the spectral evolution from an isolated molecule to the cluster and the Kondo lattice is discussed by the competition of the Kondo effect and antiferromagnetic Rudermann-Kittel-Kasuya-Yosida coupling between the molecular spins.
We have investigated electromigration process at metal nanojunctions by introducing a novel spectroscopic approach. When the junction voltage exceeded certain critical values, conductance showed successive drops by one quantum conductance, corresponding to one-by-one removal of metal atoms. The observed critical voltages agree with the activation energies for surface diffusion of metal atoms. This fact clearly indicates that the elementary process of electromigration is the self-diffusion of metal atoms driven by microscopic kinetic energy transfer from a single conduction electron to a single metal atom. This new finding can be applied to reproducible formation of atomic-spacing gaps for single molecule junctions and also to the failure-tolerant interconnects for VLSIs.
The bottom-up methods have recently attracted much attention as the fabrication techniques of nanoscale devices. Self-assembled monolayer (SAM) can be a basis to construct nanoscale structures, and many researchers have challenged to fabricate multilayer systems with self assembly. Among a variety of the preparation methods of self-assembled multilayer structures, the stepwise coordination is one of the most attractive methods because it allows us to construct complex oligomer wires quantitatively and regularly using very simple processes. Our main research targets are the fabrication of π-conjugated bis(terpyridine) complex oligomer wires and their electron transport properties. Linear and branched wires were constructed by the stepwise coordination method, and their electron transport mechanism was analyzed by potential-step chronoamperometry. Additionally, it was found that bis(terpyridine) complex oligomer wires have the excellent long-range electron transport ability and we can control it by changing combination of surface anchoring molecules, metal ions and bridging ligands.
Electrochromism is the phenomenon displayed by the materials of reversibly changing color when the electric potential is applied. We have developed new type of electrochromic materials having the polyrotaxane structure, in which the semiconductive polymer (polythiophene) is wrapped by many macrocycles [tetracationic cyclophane, cyclobis (paraquat-p-phenylene)]. Compared with the reference polythiophene without macrocycle, the polythiophene polyrotaxane thin film has more homogeneous surface and brighter color, and shows larger color contrast and faster response in the electrochromic process. These results arise from two factors based on the polyrotaxane structure. One is the weakened molecular interactions between the polythiophene chains by the macrocycles. The other is the counter anions of the tetracationic cyclophane which provides the pathway for the migration of the counter anion between the electrolyte solution and the film in redox process. These results indicate that the encapsulation of the semiconductive polymers by the macrocycles is one of the powerful methods to enhance the functionality of the materials.
The electrical conductivity of poly (3,4-ethylenedioxythiophene) doped with poly (4-styrenesulfonate) (PEDOT/PSS) was significantly improved by two orders of magnitude upon addition of ethylene glycol (EG). It was found that the EG was crucially important for (i) crystallization of PEDOT molecules, (ii) removal of the insulating PSS from the surface of the PEDOT/PSS particles, which improved both intra- and inter-particle transfer of charge carriers. Furthermore, Schottky diode, field-effect transistor, microfiber, and soft actuator were fabricated utilizing the highly conductive PEDOT/PSS by means of line patterning, wet-spinning, and cast techniques, respectively.
A few micro liters of 10-2 wt.% ionic liquid diluted by ethanol or acetone dropped onto insulating samples made it possible to observe scanning electron microscope (SEM) image at 5000 magnification, which is as clear as Pt-Pd sputtering. SEM-EDX analysis of sulfur is possible when using sulfur-free ionic liquid diluted down to 10-2 wt.%. Quantitative analysis using the ZAF method is also possible for insulating samples.
We developed a 85o-high-angle inclined specimen holder which enabled the specimen surface to be irradiated by both electron and ion beams at the glancing incidence. We have investigated the high depth resolution Auger depth profiling analysis with the inclined specimen holder. In consequence, the resulting depth resolution for the GaAs/AlAs super-lattice was found to be independent of the sputtered depth. The highest depth resolution of 1.7 nm was achieved with the Al-LVV Auger peak. The Auger depth profiles of the Si/Ge multiple delta-doped layers revealed that the Ge mono-layer can be measured in-depth profiled with high sensitivity using this inclined specimen holder.